Lord Mandelson: Nick Clegg's ultimatum sealed Gordon Brown's fate

Gordon Brown's attempts to hold on to power were thwarted by Nick Clegg's demand that he step aside as part of any coalition between Labour and the Liberal Democrats, it was reported today.

The Lib Dem leader informed the then prime minister to his face that a Lib-Lab alliance was only possible if Mr Brown bowed out.

According to Peter Mandelson's memoirs, serialised today in The Times, former prime minister Tony Blair also told Mr Brown after May's inconclusive general election that the public would not accept him remaining in office.

After further calls for him to go from other senior Lib Dems, he announced he was resigning as Labour leader to allow coalition talks with the Lib Dems to continue. Ultimately the Lib Dems formed a coalition government with the Tories.

According to the account of Labour's coalition negotiations after the May 6 poll, Mr Brown went so far as drawing up a proposed Lib-Lab cabinet featuring Mr Clegg, Vince Cable and Lord Ashdown as he plotted how to keep David Cameron out of Number 10.

But Mr Clegg, now the Deputy Prime Minister, told Mr Brown that he had to quit if there was to be a deal between their two parties.

Lord Mandelson records in his book The Third Man that, at a meeting in the Prime Minister's Commons office on the Sunday after the election, Mr Clegg said: "Please understand I have no personal animosity whatsoever.

"But it is not possible to secure the legitimacy of a coalition and win a referendum unless you move on in a dignified way."

Mr Brown and Lord Mandelson had used a secret tunnel between Number 10 and the Ministry of Defence to avoid attracting attention on their way to the meeting. Their car picked them up at the MoD to take them to Parliament.

The then prime minister is said to have avoided giving a straight response to Mr Clegg's ultimatum during the meeting, at which Danny Alexander - now the Chief Secretary to the Treasury - was also present.

It was not until the following day, after conversations with other Lib Dems and his predecessor Mr Blair, that Mr Brown resolved to resign.

According to Lord Mandelson, Mr Blair backed the principle of a Lib-Lab coalition but told Mr Brown he could not be involved.

"Tony told him and me that the public would simply not accept Gordon remaining," he writes.

Mr Brown accepted that he had to quit but did not want to look like he had been forced out, it is said.

Lord Mandelson quotes him as saying: "I have been humiliated enough."

The former business secretary also claims that Mr Brown eventually resigned as prime minister before the Tories and Lib Dems had finalised an agreement so that it was not dark as he left Downing Street for the last time.